The present invention relates to spray drying technology applicable within a broad range of industries, e.g. the pharmaceutical, chemical and food industries.
The term spray drying is here used in a broad sense as meaning not only processes for transforming a solid dissolved or suspended in a liquid into a powdery, possibly agglomerated material, but also processes in which an essential purpose is to agglomerate a particulate material by spraying and drying a liquid thereon.
Within the area of spray drying substantial improvements have been developed and implemented within the last many decades. A standard textbook on spray drying technology is Masters, Keath: Spray Drying Handbook, 5th edition, (Longman Scientific and Technical 1991), incorporated herein by reference.
Modifications of the basical spray drying process which are of special relevance in connection with preferred embodiments of the present invention include the application of an internal stationary fluidized bed in the bottom portion of the spray drying chamber as well as the arrangement of a filter in the very spray drying chamber to retain particles in the chamber otherwise removed therefrom entrained in the stream of spent drying gas exhausted from the chamber.
A process and an apparatus utilizing a fluidized bed in the bottom of the spray drying chamber is described i.a. in U.S. Pat. No. 5,632,100 (Hansen). In the embodiments described in said US patent, various types of particle collecting equipment are used for treating not only the spent drying gases withdrawn from the drying chamber but also to treat particle-loaded gas streams resulting from after-treatment of the product particles recovered from the drying chamber in fluidized bed apparatuses or gravitational classifiers.
WO 97/14288 suggests incorporation of special, rigid gas filters into the very spray chamber whereby substantial advantages may be obtained, viz. improved conditions for particle agglomeration in the drying chamber and a reduction of the requirement for particle collecting equipment such as cyclones and filters, including electrostatic filters and fabric filters for treating the spent drying gases from the drying chamber.
Said WO 97/14288 discloses an embodiment in which a product recovered from a first stationary fluidized bed in the bottom of a spray drying chamber having internal, rigid gas filters is subjected to an after-treatment in an annular fluidized bed encircling said first stationary, fluidized bed and having a common ring-shaped partition wall therewith. The gas with entrained fine particles from said annular fluidized bed is introduced into the drying chamber through horizontal slits in short distance above said first fluidized layer, and subsequently passes the gas filters when leaving the chamber together with spent drying gasses, whereby said fine particles collect on the filter surfaces.
A more detailed disclosure of a spray dryer without internal filter but having a fluidized bed in the bottom of the drying chamber encirled by an annular further fluidized bed and slits for passing particle-loaded gas from the annular bed to the drying chamber, is presented in EP 749560 B1.
However, these embodiments have some drawbacks because the presence of said slits, forming almost a ring in the lowest part of the walls of the conical bottom portion of the drying chamber or just below these, interfere with the first fluidized bed by preventing the desired spouting function thereof. A spouting of material of the first fluidized bed up on the conical walls is essential to keep these free of deposited sticky material. The slits with the upward gas flow therethrough prevent proper return of the spouted material into the first fluidized bed, and involve a risk of relatively moist material dropping down into the annular fluidized bed.
Therefore, it is conventional to use one or more independent, separate apparatuses for the after-treatment.
Whether the after-treatment is a supplementary drying, a cooling, a classification, an agglomeration or a separation, a particle-loaded gas stream is formed, from which the particles must be separated to recover values therein or to avoid environmental pollution. Therefore, the particle-loaded gas stream withdrawn from the separate after-treatment apparatus is in the prior art processes passed through collection means serving solely the purpose of separating the particles from said gas stream. Also special measures had to be taken to incorporate the separated particles into the main product stream or for utilizing them in other applications.
These separate after-treatment apparatuses as well as the means for collecting the particles from the gas stream therefrom and the means for handling of the collected particles increase the total apparatus costs and also increase the space requirements.
Thus it is an object of the present invention to provide a process and an apparatus, which is simplified by omitting the collector means for collecting the particles from the gas from the after-treatment step and which can be performed in a plant being very compact since the after-treatment equipment is closely united with or built together with the fluidized bed in the bottom of the spray drying chamber, without presenting the drawbacks of the above discussed embodiment of the apparatus disclosed in WO 97/14288.
It has turned out that said drawbacks can be overcome by introducing the particle-loaded gas stream from the after-treatment into the spray drying chamber by pipe means, thereby avoiding the ring-forming slits causing problems as described above. This can be done without disturbing the primary drying gas flow and the drying and agglomeration processes in the drying chamber, and also without decreasing the quality of the resulting product.
Thus, the invention deals with a process for spray drying a liquid to produce an agglomerated product by atomizing the liquid into a drying chamber to form droplets, introducing a first stream of drying gas downward into the upper part of said chamber to partially dry the droplets, introducing a second stream of drying and fluidizing gas upward in the bottom portion of the chamber through a perforated plate to maintain a fluidized layer of particles on said plate, withdrawing a stream of spent drying gas comprising gas from said first and said second stream of gas from the chamber through particle collecting filter means thereby retaining particles on the surface of said filter means, releasing said retained particles from the filter means, to allow their contacting other particles in the chamber for agglomeration and transfer into a first fluidized layer in the chamber, from where the particles after further drying pass to a second fluidized layer intimately united to and encircling or immediately beneath said first fluidized layer for an after-treatment comprising at least one treatment selected from after-drying, cooling, classification including dust removal, agglomeration, coating and separation and withdrawing dust-loaded gas from a location above said second fluidized layer by pipe means and by said means introducing it into the drying chamber from where said gas passes through the particle-collecting filter inside the chamber before leaving the chamber together with said spent drying gas.
The spray drying process may be of the kind in which the liquid being atomized contains a dissolved or otherwise dispersed solid, which after the drying constitutes the final product.
Alternatively, the process may be characterized in that a particulate solid is injected into the chamber for agglomeration promoting contact with the atomized liquid droplets or moist particles formed by partial drying thereof, and in that solids contained in the liquid show adhesive properties when dried and/or the liquid comprises a solvent evoking stickiness of one or more components of the particulate solid.
It is a feature of the process according to the invention that the pipe means for introducing the particle-loaded gas stream into the spray drying chamber, in contrast to the prior art compact constructions provides flexibility as to the locations for the introduction. In a preferred embodiment the introduction takes place into the upper portion of the drying chamber.
The invention also deals with an apparatus for performing the above specified process, having a drying chamber, an atomizing device in said chamber connected to a source for liquid to be spray dried, a drying gas disperser in the upper portion of the chamber, a perforated plate in the bottom portion of said chamber, a plenum below said plate for providing an upward flow of fluidizing and drying gas through the perforations in said plate, sufficient for maintaining a first bed of fluidized particles on said plate, a particle-collecting filter inside said chamber, means for withdrawing a stream of gas from said chamber through said filter, a second fluidized bed intimately united to and encircling or immediately beneath said first fluidized bed, uniting means for transferring particles from the first fluidized bed existing on said plate during the operation of the apparatus, to said second fluidized bed serving as after-dryer, cooler, classifier, agglomerator, coater and/or separator for said transferred particles, which second fluidized bed during the operation thereof emits a stream of gas entraining small particles, characterized in having pipe means for introducing said stream of gas with entrained fine particles into said drying chamber at a location upstream of said filter.
In preferred versions of the apparatus, the integrated filter is located in an upper portion of the spray drying chamber.
Most experiences have been obtained using drying chambers having a lower portion of downwardly decreasing horizonal cross-section area, the bottom part of which portion accommodates the perforated plate and the fluidized layer and the filter being located above the slanted wall of said lower portion.
The amount of fine particle-loaded gas produced in the after-treatment may be large, e.g. corresponding to up to 50% of the amount of gas introduced into the drying chamber as drying and fluidizing gas. To avoid that such a large amount of gas disturbs the flow pattern existing in the chamber, which flow pattern is essential for obtaining the desired operation and product qualities, it is preferred that the pipe means connecting the after-treatment zone and the drying chamber debouches in an upper portion of the latter on a level with the filter.
In such an embodiment the filter may comprise at least one essentially vertical member of which at least a part is between the atomizer and the location in the upper portion of the chamber for debouching of said pipe means for introducing the gas stream with entrained fine particles.
In a specific embodiment of this last-mentioned version of the apparatus the filter comprises a plurality of vertical cylinders encircling the atomizer device, the pipe means for introducing the gas stream with entrained fine particles debauching at least at one location outside the one encircled by the cylinders and preferably at a distance to the nearest of said cylinders no less than 1.5 times the diameter of this cylinder.
By this arrangement it is avoided that the fine particles from the external after-treatment unit contact the hot areas in the drying chamber. This is essential since said fine particles are, due to their lower moisture content, more at risk of being heat-damaged than the average particles present in the drying chamber.
This last-mentioned embodiment furthermore has the advantage that the risk for damage of the filters due to abrasion is minimized, by avoiding impingement by very fast particles on the filter surface. Furthermore, a uniform building-up of particles on the filter surface is obtained.